Reactions involving electron transfer at semiconductor surfaces. Part 11.—Oxygen isotope exchange via photoinitiated R1, R0 and place exchange processes on ZnO and TiO2

Abstract

Comparisons are made between the changes in isotopic composition of isotopically pre-equilibrated (i.p.eq.) and isotopically non-quilibrated (i.n.eq.) gaseous oxygen in contact with prereduced or preoxidised samples of ZnO and TiO₂ at room temperature. In the absence of illumination a place exchange (p.x.) process predominated in the oxygen isotope exchange (o.i.e.) detectable at low pressures, ca. 8 × 10^–3 Torr, of i.p.eq. O₂ upon contact with preoxidised samples, whereas a homophase, R₀-type o.i.e. process predominated for i.n.eq. (¹⁶O₂+¹⁸O₂) contacted with prereduced samples at pressures of ca. 10^–1 Torr. The latter R₀ activity was removed by preoxidation but light restored it with quantum efficiency > 6 for pure ZnO and > 30 for lithium-doped ZnO. A mechanism is described to account for the correspondingly high turnover achieved on each site photoactivated by light. For i.p.eq. O₂ exposed to intense illumination in contact with prereduced or preoxidised ZnO, a heterophase R₁-type process, accompanied by a faster R₀-type process, predominated and reasons for this are considered. Residual hydroxyls affect the ratio of o.i.e. processes on TiO₂ surfaces.